This project deals with the mechanisms that allow cells to keep their water content at a constant level. It focuses on the receptors that perceive changes in cell water, the membrane transporters that correct perturbations in cell water, and the signaling systems that link the receptors with the transporters. The aims of the project are to identify the receptor(s), to study how volume-activated transport systems are coordinated, and to study how volume is regulated in vivo. Red cells are selected for study because of their anatomical simplicity. The experiments involve measurements of water, ions, and metabolic intermediates in red cells plus radioisotopic labeling and separation of red cell membrane proteins. The in vivo work entails the administration of an antimetabolite, hydroxyurea, to experimental animals. The red cells of human subjects who are being treated with hydroxyurea for myeloproliferative disorders will also be studied. The clinical importance of this work is that changes in cell water content are a common and potentially lethal manifestation of many disease states, including brain injury, sickle cell disease, and acute renal failure.